A. Field of the Invention
The present invention relates to pressure transducers for medical use. More particularly, the present invention relates to devices and methods used for calibrating a disposable pressure transducer adapted for medical use in direct measurement of blood pressures and the like.
B. The Prior State of the Art
One of the most frequently measured conditions of a patient undergoing an evaluation, diagnosis or treatment is the patient's blood pressure. There are generally two types of methods used for measuring and/or monitoring blood pressure, namely, indirect blood pressure measurement, and direct blood pressure measurement. Indirect blood pressure measurement techniques are based on non-invasive procedures such as using a pressure cuff and a stethoscope. Direct blood pressure measurement techniques, on the other hand, are made by using invasive techniques which permit intracorporeal measuring and monitoring of blood pressures.
Notably, when diagnosing and treating critically ill patients, direct blood pressure measurements are almost always preferred over indirect blood pressure measurements due to several factors. First, use of direct blood pressure measurement greatly increases the accuracy of the blood pressure reading. Second, direct measurement facilitates the continuous monitoring of a patient's blood pressure as opposed to intermittent monitoring. Third, direct blood pressure measurement enables rapid detection of any change of cardiovascular activity, which may be of particular importance in emergency situations. Fourth, direct blood pressure measurement can also readily be used to measure and monitor a patient's blood pressure at specific internal locations, such as within the chambers of the heart. Because of these and other advantages, therefore, direct blood pressure measurement and monitoring has become a routine procedure in treating critically ill patients.
Direct blood pressure measurement and monitoring typically involves catheterization. A hollow needle or cannula is first inserted into a peripheral blood vessel. For example, in the case of monitoring arterial blood pressure, it is inserted into a radial artery. If venous blood pressure is to be monitored, the needle is inserted into the antecubital, radial, tubular or subclavian vein. A catheter is filled with a sterile saline solution and de-bubbled. The catheter is then threaded through the needle and into the particular blood vessel. The catheter may then be guided along the blood vessel until the tip of the catheter is positioned at a particular point within the body at which it is desired to make the blood pressure measurement. With the catheter thus in place, the needle may be withdrawn and the entry site of the catheter is taped down.
Prior to positioning the proximal end of the indwelling catheter within the patient as described above, the distal end of the catheter is connected to pressure tubing that in turn is connected to a pressure transducer. The catheter is generally also connected to a suitable continuous flush device or heparin drip to prevent clotting around the tip of the catheter. The pressure transducer is in turn electrically connected to a monitor and/or other output devices near the patient's bedside which display the patient's blood pressure waveform using analog and/or digital readouts.
When setting up the blood pressure measurement system as described above, it is important to prime the catheter and the tubing with the sterile solution in a way so that any air bubbles within the catheter and tubing are removed, thus providing a continuous fluid column that runs from the tip of the catheter to the pressure transducer. Then, when the catheter is positioned within the patient's blood vessel, as the patient's heart pumps blood, periodic pressure pulses are transmitted through the patient's blood vessel and along the incompressible fluid column in the catheter to the pressure transducer. The transducer generates electrical signals representing the pressure pulses, and those signals are then amplified and displayed on the appropriate monitor and/or output devices. Usually, in such a system a display monitor is used to show the patient's blood pressure as a function of time. This type of display being commonly referred to as the blood pressure waveform. This blood pressure waveform is used by medical personnel to appropriately diagnose and treat the patient.
One of the most important components of the abovedescribed blood pressure monitor system is the pressure transducer. Significantly, the accuracy and the precision of the pressure transducer set an upper limit to the quality of the blood pressure data which can be obtained. Accordingly, pressure transducers for this type of application have been developed which have a high degree of reliability, sensitivity and accuracy. Typically, transducers for this type of application are comprised of piezoresistive semiconductor integrated circuits or "chips." These transducers have a thin diaphragm which, on one side, is in fluid communication with the sterile fluid column contained in the catheter and tubing. This thin diaphragm is deflected by the pressure pulses which travel through the fluid column in the catheter and tubing. As the diaphragm is flexed by virtue of the pressure pulses, the electrical resistance of the diaphragm changes in proportion to the amount by which the diaphragm is flexed, and appropriate electronic circuitry generates an electrical signal representing the pressure exerted on the diaphragm. In this way, the blood pressures are transformed into a suitable electrical signal or waveform.
In order to assure accuracy of pressure measurement, the transducer should be calibrated at a zero pressure by a process called "zeroing." Zeroing a transducer is important in that falsely reported pressures will yield inaccurate data that is unreliable for purposes of evaluating and/or diagnosing a patient's condition. Calibrating a transducer may be done frequently, depending on the reliability of the particular transducer being used. Reliability is affected by the transducer's propensity to produce drift or change in its output signals incident to stress from excess pressure or from thermal changes.
Transducers can be calibrated from either the front or the backside, the front side being the side that is in fluid communication with the sterile fluid column of the catheter and tubing. The backside is the side of the transducer diaphragm that is not in contact with the sterile fluid column. By exerting a known positive pressure on the front side, a display device can be read to determine if the transducer is reading at the known pressure. Conversely, by exerting a known negative pressure or vacuum on the backside of the transducer, an effect equivalent to a known positive pressure on the front side is achieved, and again, the display device can then be read. In either technique, the accuracy and need for calibration is judged by the degree of deviation of the actual transducer pressure reading as compared to the applied and known pressure. Adjustments to the monitor can then be made as necessitated and dictated by the reading, to calibrate the transducer.
There are various difficulties and hazards with calibrating a transducer by using the front or patient side that is in direct contact with the sterile fluid column that is intravenously situated in the patient. Ordinarily, calibration is done by an instrument that introduces pressure to the transducer by a fluid medium such as air. Difficulties and hazards of patient side calibration arise from the potentiality for entry of air and/or contaminants into the patient's intravenous system. The entry of air and contaminants are a potential cause of embolisms and infection. Thus, it is desirable to avoid transducer calibration on the patient side of the transducer.
Also known in the art is backside, or non-patient side, calibration. Backside calibration avoids the undesirable aspects of patient side calibration by means of applying negative pressure to the backside of the transducer, which is isolated from the intravenous fluid column. Backside vacuum pressure is equally effective as patient side positive pressure for calibrating the transducer, while avoiding the complications of non-sterility and air emboli, which are known hazards in front side calibration techniques.
One type of backside calibration apparatus is disclosed in U.S. Pat. No. 4,610,256 to Wallace. A cable electrically connects a pressure transducer to a microprocessor, which in turn numerically quantifies the signals output from the transducer. In addition to electrically conductive wires, the cable has an air space running along its length for communication of vacuum pressure through the cable to the backside of the transducer, for the purpose of effecting calibration of the transducer. While the Wallace disclosure uses backside calibration to avoid the aforementioned hazards inherent to patient side calibration, Wallace is not without drawbacks in its disclosed technique.
The cable used to introduce the negative calibration pressure is one source of such drawbacks in that such a cable is expensive to manufacture. The cost of manufacturing mitigates against the economics required of items intended to be deposed of after use. In addition to the manufacturing expense, the cable is also generally difficult to manufacture. Further, attachment of the cable to the transducer is manually difficult, in that if the cable is secured too tightly the air space provided through the cable may be altogether pinched off, thus precluding the backside pressure required for calibration. Thus, to avoid this result, the cable connection must be loosely connected. Of course, a loosely connected cable in itself has inherent problems. Should the cable be inadvertently bumped, pulled upon or jarred, the electrical connection to the transducer can be disrupted.